An outdoor warning siren system is just that, an OUTDOOR WARNING SYSTEM. It should not be depended on as the primary warning system for indoors. New construction methods, terrain, location and storm related power outages can all reduce the sound level of the sirens to a point they cannot be heard indoors. Other emergency managers have echoed this message.
That being said, the siren system does provide an important adjunct to alert people who are outdoors to an approaching hazardous weather condition.